Technical FAQ: Carbon rims vs. car exhaust, short vs. long braking

Car exhaust heat and carbon wheels

Dear Lennard,
I’ve been an early adopter of discs for ’cross and every day commuting use — I ordered a XCR tubed ’cross frame with an ENVE disk fork not long after the fork was released. We do end up having our share of mud days in XC but also CX. I’ve consistently found the advantage in disk brakes in ’cross to be coming into a sharp u-turn or series of turns after a long straight, for example, reaching 40+ kph. I’ve seen guys on [cantilever brakes] pop after having to repeatedly close the small gap due to the braking differences.

I have ordered a couple sets of the new kettle carbon discs to try. One is for some ENVE tubulars that see daily use for commuting and training (challenge 27mm rubber), CX racing, and also XC MTB racing. The other set is for a MTB. But after reading your article, I may try them on my dedicated mud CX wheelset as they don’t seem to have many — if any — holes in them. They are pricey for sure but they may fit the bill for those using disks and racing often in the wet if they do in fact hold up.

I also thought I would let you know of something that happened this weekend as I haven’t seen or read anything about it. I put my bike on my rear rack because I had the kids in the car and couldn’t be bothered removing the front wheel. I took a short drive to my LBS as a bike needed a piston kit. It was a very hot day and the drive was about 20 minutes, with half at freeway speeds. Upon arrival at the shop, I found my ENVE UST wheels were cooked! I have an insurance claim in at the moment but with all the hype about the heat resistance of carbon rims, it seems they really aren’t that great. I’ve had many alloy rims in the same position on the same rack with no hint of damage and there was also no damage at all to the tire.

It was definitely exhaust heat on the front wheel, not impact damage from driving — the rear must have been far enough away to not get damaged. I spoke with ENVE about increasing awareness of the issue. I’m not faulting the wheel, just saying that with rear mount racks being ubiquitous, one doesn’t hear much or see warnings about the hazards of exhaust heat to carbon wheels.
—James

Dear James,
It never occurred to me to be concerned about overheating carbon rims by hanging them in front of a car’s exhaust pipe. So here’s the obligatory warning — be careful about hanging a bike with carbon rims on a rack mounted on the back of your vehicle. Make sure that the wheels are far from the exhaust pipes!
― Lennard

Short vs. long braking: Companies weigh in

Dear Readers,
In my April 30 post, I answered a question from John about short vs. long braking and latex or butyl tubes for mountain riding on carbon clinchers. I promised to get more feedback from manufacturers about it. Survey says: I was right about preferring shorter, harder braking, and I was wrong that the type of inner tube is unimportant. I think all of the answers are interesting, and each looks at it from a slightly different angle. See below.
― Lennard

From Zipp:

For any given stop, say coming to a stop at the bottom of a hill, a stoplight, etc., there really isn’t much difference. You have to bleed energy and doing it over a short time or longer time doesn’t change the temperature much by the time you stop. So for 98 percent of riding, the answer is that it really doesn’t make much difference.

However, on a long or tricky descent, giving the rims and pads periods of cooling is very beneficial to keeping the system temperature under control. Most important is to not drag the brakes for prolonged periods of time as this continually heats the rim, pads, tube and tire. In our internal testing, we have achieved surface temperatures approaching 700 degrees Fahrenheit after 5-6 minutes of prolonged heavy braking (simulating a 300-pound guy descending a 20 percent grade for 5+ minutes, so not a likely real world situation). Releasing the brakes for even a few seconds can reduce the temperatures on the rim by over 100 degrees, whereas keeping the brakes applied, even lightly, will continue to heat the system. Again, whether you begin to slow 75 meters before the hairpin or 60 meters before isn’t very critical, but riding the brakes the entire way down the mountain isn’t good for any of the components in the system.
—David Ripley
Technical PR Manager
Zipp Speed Weaponry

From DT Swiss:

In our experience, the shorter, more aggressive braking method is preferable. While the heat at the rim/pad interface will be higher during braking, the additional time between brake applications will allow for more cooling of the rim as compared to a prolonged braking method. Put another way, when brakes are used for a longer time at lower power, there is less time for the rim to cool off in between brake applications.

Decelerating from one given speed to another is going to require equal energy input, regardless of braking method. However, having an increased cooling period between braking will let the system cool off more, decreasing heat build up.

Since we are talking about carbon wheels specifically, it is important to keep other elements in mind. Firstly, tire pressure and size play a significant role here. If a rider knows he or she will be doing a lot of descending, it may be a good idea to lower tire pressure a bit before the ride. As rims heats up, tire pressure will increase, possibly past the recommended limits for the rim or tire. Also, a larger tire at a given pressure will exert more outward force on a rim, so it’s key to drop pressure a bit as one increases tire size. Finally, latex tubes are far more susceptible to flatting from heat, often without the tire ever coming off the rim (see Tony Martin’s infamous flats at last years TDF), and as such should never be used in our carbon wheels (I think other manufacturers have a similar recommendation).
—Steven Sperling
DT Swiss, Inc.

From Hed Design:

We have been measuring rim heat up for a while now. The worst case, this is real world with a temperature reader, is a long, feather-type braking. This warms up the rim [more] than a hard pulse stop — it spikes an already warm rim. Carbon is a good insulator, so when it can’t dissipate the heat, it slowly collects it and holds it. FYI — we cannot get an alloy rim to heat up over 250 degrees Fahrenheit, carbon rims we’ve gotten over 450 degrees. We spent a couple of years getting our carbon brake walls to take the heat, using high temp resins and a higher curing progress.

The next problem is solving the rubber in the tire and tube from degrading. I think the rubber used in tubes and tires is vulcanized at around 320 degrees. So when tires and tubes see more than this they start to break down. It might take repeated elevated temps to do this, the first hard stop may not, but over cycles the tube can start to fail, even though the tire hasn’t. My advice, if you know you have spikes of heat into your carbon rims, is to at least replace the tubes and inspect the tire bead to see if it has become soft, gummy, or degraded and replace if needed. We are still working on the problem.
—Steve Hed
Founder, Hed Design

From Reynolds:

The worst type of riding style for carbon wheels is the “rear brake dragger.” Prior to our CTg laminate, the typical carbon rim heat failure was almost always the rear wheel; rarely did we ever see a front wheel with heat failure. Now, with the CTg laminate we don’t really get many heat failures at all, not even rear wheel failures.

To answer your question directly … Yes, for sure, shorter, more powerful braking produces less heat build-up than prolonged braking. We’ve known this for a long time to be the case.

I’ve had similar discussions with other manufacturers and we all have the same experience. I reference a conversation with Jorg Ludwig not too long ago when he was telling me that he, Mario Cipollini, and other pro riders never had a heat failure with their team’s superlight carbon wheels, but that in the commercial marketplace his experience is the same as mine, that the heat failures with those same wheels are mostly rear wheels as a result of dragging the rear brake.
—Paul Lew
Director of Technology and Innovation Reynolds Cycling

From ENVE Composites:

Heat buildup in the rim will have more to do with average speed during the duration of the descent than the method of braking. For the duration of a descent, coasting and then braking into the corners usually results in a higher overall average speed. Higher average speeds result in lower rim temperatures because of the following.

– At higher speeds, much more energy is dissipated by aerodynamic drag than at slower speed. Higher speeds shift the balance of energy dissipation to aerodynamic drag, and away from the rim and brake pad.
– Convection of heat off the rim is accelerated by the higher air speeds.

One scenario where the braking style will change heat buildup in the rim, even if the same average speed is maintained:
-Dragging brakes tends to glaze pads, and glazed pads don’t shed material under braking. Shedding of pad material is of significant benefit to keeping the pad/rim system temperatures under control. Glazed pads run significantly hotter than pads that haven’t glazed.

We test braking of our wheels at a steady speed and power, as that was what we found to be the riding style that resulted in the highest rim temperatures during ride testing.

Regarding internal rim temperatures:
-Deep rims tend to run cooler than shallow rims. The deep sidewalls act as a heat sink, drawing heat away from the brake track.
-In our testing, aluminum rims run hotter internally than carbon.
-Latex tubes are more sensitive to heat than butyl. Under prolonged braking, any slight irregularity in the tire bed (like the edge of a rim strip, or the transition from tire bead to tire bed) may result in sudden deflation of the latex tube. We’ve seen this in both carbon and aluminum rims. Latex may work really well for riders who aren’t faced with long descents, but we can’t say that they are safe for all use scenarios, so we can’t recommend using latex tubes.
—Brent Pontius
Test Engineer
ENVE Composites

From Mavic:

I double-checked with our R&D team. The heat would be the same in both cases.

The difference is that in shorter braking, the temperature will not remain high for a long time. With prolonged braking, the temperature remains high for a longer period of time and is what causes issues on carbon rims, inner tubes, brake fluid, etc. (depending on what brake system you use).
—Maxime Brunand
Mavic Road Product Line Manager
Annecy Design Center, France

From Campagnolo:

The longer the brake pad is in contact with the braking surface, the more friction will be transferred to heat. Even a dragging/rubbing miss-aligned brake caliper can cause heat build up. On long descents, dragging or “feathering” the brakes actually causes more heat build up than quick, short uses.
—Daniel Large
North America Technical Service
Campagnolo North America Inc.

From Shimano:

Steady drag of brakes produces more heat buildup, because shorter, high-intensity braking allows heat dissipation during non-braking time.
—Wayne Stetina
VP R&D
Shimano American

From Dash Cycles:

For slowing down on descents, I always recommend against prolonged braking. It can depend on the quality of the rim and brake pad, but shorter, more powerful braking will typically produce less heat buildup.
—Tom Guerin
Founder, Dash Cycles